1. Field of the Invention
The present invention relates to transmission systems in human-powered, chain driven vehicles. More particularly, it relates to front planetary and rear-derailleur bicycle transmission systems. Still more particularly, it relates to an enclosed bicycle drive train including a front planetary and rear-derailleur transmission system.
2. Description of the Related Art
The Both external (i.e., exposed) and internal (i.e., enclosed) drivetrains are common in the modern field of bicycle drivetrains. External drivetrains typically employ parallelogram-type derailleurs, a chain, and sprockets. Parallelogram-type derailleurs swing through various ranges of motion and are therefore difficult to completely enclose. The rear parallelogram-type derailleur bolts to the outside of the bicycle frame near the rear wheel hub. From this location, the parallelogram of the rear derailleur can swing even further outboard.
External drivetrains have the advantage that the number and overall range of drive ratios is not limited by the requirement that the mechanisms fit inside any wheel hub or enclosure. Typically, external drivetrains employ two or three chain rings (front sprockets) cooperating with a front derailleur. The front shifting system gives the major ranges of gearing, and a rear derailleur cooperating with rear sprockets provides the fine tuning with smaller increments of drive-ratio shifting. The rear derailleur typically controls the chain destination among seven or eight rear sprockets associated with a rear wheel. The overall drive ratio variation typically approaches 4:1.
Internal drivetrains usually take the form of an internally geared hub. The hub may be mounted near the bottom bracket using a jackshaft but is usually integral with the hub of the rear wheel. Internal gear hubs typically use planetary gearing and normally provide from three to nine speeds. Internal gear hubs have been produced with as many as twelve or fourteen speeds, but the additional weight associated with the rear wheel makes these internal gear hubs less than ideal.
The weight of a rear hub with internal gearing is acceptable for pavement riding, but undesirable for off-road riding. This has given rise to attempts to relocate it to a jackshaft near the bottom bracket, closer to the center of gravity of the bicycle. Unfortunately, the planetary gearing and jackshaft bearings reduce the efficiency of the system compared to a standard external drivetrain.
There are many tradeoffs between external and internal drivetrains. External has good gearing range and good efficiency, but is exposed to wet and mud, which can drastically reduce the efficiency and increase wear on the drivetrain. Also, the exposed drive train is vulnerable to derangement due to impact or even a small object such as a twig getting stuck in the chain, which can easily obstruct the idler pulleys, and cause the rear derailleur to break off.
Improvements to external drivetrains, such as indexed shifting and special features in chains and sprockets to promote clean and prompt shifting, have made the external drive train shiftable under load in most circumstances. The only condition under which a typical modern external drivetrain shifts poorly is when the front derailleur is shifted under load. The problem is inherent that the front derailleur is set up to shift the loaded side of the chain, in contrast to the rear derailleur, which manages the slack side of the chain, making rear shifting generally much easier.
Internal gear hubs are of course enclosed, and the mechanisms live in an oil or grease bath substantially free of contamination.
Internal gear hubs change ratios without the need to alter the chain path between multiple sprockets. The chain can easily be enclosed in a reasonably slim chain guard. Therefore, the shifting mechanism, sprockets, and chain are completely protected from the elements and from contaminants. This makes these bicycles very suitable for commuting in all conditions and storing outdoors.